Recent preclinical studies have demonstrated that adoptive T cell transfer into a host rendered lymphodepleted followed by vaccination and IL-2 therapy favors differentiation of memory cells capable of curing relatively advanced solid tumors. Although several mechanisms have been suggested, recent studies have demonstrated that these mechanisms might not be the principal means by which lymphodepletion augments adoptive T cell therapy. Therefore, a better understanding of the mode of action of lymphodepletion would open new avenues for improving anti-tumor memory responses of adoptive T cell therapy. We recently have found that CTX treatment induces the expansion of immature DCs during the lymphoid recovery phase from day 9 to day 16, peaking on day 12. These DCs demonstrated normal phagocytic ability in vitro and antigen uptake in vivo. Administration of the TLR3 agonist poly(I:C) at the peak of DC expansion induced a rapid inflammatory milieu that was associated with significant increases in the numbers of activated DCs in lymph nodes. Using the pmel- 1 TCR transgenic mouse model, in which CD8+ T cells recognize gp100 melanoma peptide, we have demonstrated that priming of the CTX-treated mice with gp100 peptide plus poly(I:C) at the lymphopenic phase followed by boosting at the recovery phase (the peak of DC expansion) resulted in significant increases in the number of activated DCs in lymph nodes with a temporal increase in the expansion of pmel-1 cells, resulting in a robust therapeutic anti-tumor effects toward a bulky (advanced) tumor of the poorly immunogenic B16 melanoma. Therefore, we hypothesize that providing the inflammatory milieu induced by TLRs agonists optimally timed to be in combination with a unique expanded presence of post CTX DCs can lead to robust long-lasting anti-tumor immunity. To address this hypothesis, we will compare the anti-tumor effects of our treatment modality consisting of vaccination with hgp100 peptide and poly(I:C) to those of the established modality consisting of vaccination with ex vivo hgp100-pulsed DCs and IL-2. We will then determine if overlapping (by addition or substitution) of the adjuvant components of our modality to those of the established modality can results in a much higher anti-tumor responses. Finally, we will define the intrinsic and extrinsic mechanisms mediating these anti-tumor effects. We do believe that the increased frequencies of DCs during the recovery phase post CTX-induced lymphodepletion represents a novel opportunity to improve the rationale design and clinical application of chemotherapy in combination with tumor immunotherapeutic strategies. PUBLIC HEALTH RELEVANCE: Adoptive T cell transfer into tumor-bearing hosts rendered lymphopenic by irradiation or treatment with anti- cancer chemotherapeutic drugs markedly enhances the anti-tumor T cell responses, however, the mechanisms underlying these beneficial effects of lymphodepleting regimens are poorly understood. We have made the novel observation that treatment with the anti-cancer drug cyclophosphamide induces a marked expansion of dendritic cells, which play central roles in shaping the immune responses against cancer, representing a novel mechanism underlying the beneficial effects of this drug to adoptive immunotherapy. Further analysis of our observation would open new avenues for the rationale applications of anti-cancer adoptive immunotherapies.